The invention relates to channel drop filters, and in particular to a highly efficient resonator-system channel drop filter. In exemplary embodiments, photonic crystal channel drop filters are provided.
The increasing interest in photonic integrated circuits (PICs) and the increasing use of all-optical fiber networks as backbones for global communication systems have been based in large part on the extremely wide optical transmission bandwidth provided by dielectric materials. This has accordingly led to an increased demand for the practical utilization of the full optical bandwidth available. In order to increase the aggregate transmission bandwidth, it is generally preferred that the spacing of simultaneously transmitted optical data streams, or optical data channels, be closely packed, to accommodate a larger number of channels. In other words, the difference in wavelength between two adjacent channels is preferably minimized.
Channel dropping filters (CDFs) that access one channel of a wavelength division multiplexed (WDM) signal, and do not disturb the other channels, are essential components of PICs and optical communication systems. Among various devices introduced recently, resonant filters are attractive candidates for channel dropping because they can potentially be used to select a single channel with a very narrow linewidth. A schematic block diagram of a resonator-system CDF 10 is shown in FIG. 1, where two waveguides, the bus 12 and the drop 14, are coupled through a resonator-system 16 having one or more resonant cavities. The bus 12 includes an input port and output port, and the drop 14 includes a forward port and a backward port. While WDM signals (i.e. multi-frequency signals) propagate inside one waveguide (the bus), a single mode is transferred out of the bus and into the other waveguide (the drop), either in the forward or backward propagation direction, while completely prohibiting cross talk between the bus and the drop for all other frequencies.
It will be appreciated by those of skill in the art that a channel drop filter is a filter which can transfer a channel from one waveguide to another, such as dropping a channel from one waveguide to another waveguide, or adding a channel from one waveguide to another waveguide.
The performance of a CDF is determined by the transfer efficiency between the two waveguides. Perfect efficiency corresponds to 100% transfer of the selected channel into either the forward or backward direction in the drop, with no transmission or back reflection into the bus. All other channels should remain unaffected by the presence of the optical resonator.
It is also desirable to obtain on/off switching functionality in the CDF. In other words, it is desirable to be able to switch on and off the transfer of a frequency channel. Ideally, in an xe2x80x9conxe2x80x9d state, the frequency channel is completely transferred from the bus to the drop waveguide, while in an xe2x80x9coffxe2x80x9d state, the frequency channel remains unperturbed in the bus waveguide. It is conceivable to achieve such on/off functionality using frequency tuning mechanisms. However, to turn off the device, the resonant frequencies of the coupling element must be shifted out of the entire frequency range of the signal. Such a frequency shift tends to require large switching power, and is not very practical.
Accordingly, it is an object of the invention to use an absorption-induced switching effect to achieve the desired on-off functionality.
The invention provides a highly efficient channel drop filter. The filter employs a coupling element including a resonator-system between two waveguides, which contains at least two resonant modes. The resonator-system includes one or more interacting resonant cavities which in addition to being coupled to the waveguides, can also be coupled directly among themselves and indirectly among themselves via the waveguides. Each component of the coupling element can be configured or adjusted individually. The geometry and/or dielectric constant/refractive index of the resonator-system are configured so that the frequencies and decay rates of the resonant modes are made to be substantially the same in the case where absorption is not included inside the resonator-system. The filter can achieve 100% signal transfer between the waveguides at certain frequencies, while completely prohibiting signal transfer at other frequencies. In exemplary embodiments, the filter is configured with photonic crystals.
In accordance with alternative embodiments of the invention, there is provided channel drop filter devices with flat-top and straight-sidewall lineshape characteristics. These lineshape characteristics are realized by using several resonances to couple the waveguides, and by designing the relative positioning of the frequency of each resonance with respect to one another. The usage of multiple resonances also allows the design of optical switches. These devices can switch the signal from a forward to a backward direction, or from a complete to a zero transfer.
Accordingly, in one embodiment of the invention there is provided an electromagnetic field frequency filter which includes an input waveguide which carries a signal having a plurality of modes including a desired guided mode, and an output waveguide. A resonator-system is coupled between the input and output waveguides which transfers the desired guided mode to the output waveguide and allows transmission of the remaining frequencies through the input waveguides. The resonator-system includes an external sub-element that couples with the input waveguide, the external sub-element having a local mirror plane perpendicular to the waveguides, and defines at least two local resonant modes of opposite symmetry with respect to the mirror plane, the resonant modes having components which cancel in the backward direction of the input waveguide.
In accordance with yet further alternative embodiments of the invention there is provided a channel drop filter with on/off switching functionality. The switching is achieved by incorporating materials with tunable absorbing characteristics into the coupling element. When the tunable material displays minimum absorption, the frequency channel of interest is transferred completely from the bus waveguide to the drop waveguide. When the material displays maximum absorption, on the other hand, the frequency channel is not transferred, but rather remains undisturbed in the bus waveguide. The switching action can be readily achieved with either electrical or optical means.